1. Field of the Invention
The invention relates to an apparatus and method for counting sheets, particularly security documents such as banknotes.
2. Description of Related Art
It is already known to provide apparatus for counting sheets held in a stack, the apparatus comprising a set of rotatably mounted suction spindles mounted for movement past a stack of sheets to be counted, vacuum supply means connected to the spindles, whereby as a suction spindle passes the stack, a vacuum is supplied to the spindle so that the topmost sheet is deflected from its initial position; and monitoring means for monitoring the number of deflected sheets. Such apparatus is hereinafter referred to as of the kind described and is commonly referred to as a xe2x80x9cspindle counterxe2x80x9d.
Most spindle counters require a minimum pressure (vacuum) to be maintained within the system with the counting being achieved by means of external electromagnetic/photoelectric sensors which operate independently of the vacuum system provided the minimum pressure is maintained. An example is described in GB-A-2041888.
Another approach is to detect changes in the pressure or vacuum supplied to the spindles. An increase in vacuum (decrease in pressure) corresponds to a sheet being deflected and this change can be used to implement a count. Examples of such spindle counters are described in GB-A-2238411, GB-A-2238895, GB-A-2137000 and GB-A-1530652.
In some of these known spindle counters, for example those described in GB-A-2238411 and GB-A-2238895, it is necessary to index the spindles to a known position prior to the start of the count process. This is undesirable.
A further problem with systems such as that described in GB-A-2238895 is that if a spindle fails to deflect a note during a count process, the system will stop. This leads to problems in that the whole process has to be restarted.
In EP-A-0616300 we describe some developments of a spindle counter. In one aspect, a count process is only terminated when at least two spindles pass the stack without deflecting a sheet. We also describe a method of adapting the predetermined level threshold over a period of time.
Although the spindle counter described in EP-A-0616300 works well, there is a continuing need to increase accuracy, particularly at higher sheet counting rates.
For example, it is known that as the sheet counting rate is increased (by increasing the rate of rotation of the spindles) the width of the vacuum pulse caused by the sheet being held by suction on the active spindle face reduces proportionally. However, it has been determined from experience, that as the sheet counting rate is increased, the characteristics, such as sheet size, material porosity, sheet stiffness and variations which exist between the individual sheets, of the sheets within a stack, which increasingly effect the accuracy of the counting also manifest themselves not only in the signal level but also in the shape of the vacuum pulse caused by the sheet being held by suction on the active spindle face.
Furthermore, associated with increasing the count speed of spindle counters a problem can occur when it is required that the power of motor used to rotate the spindle head is not to be increased, or is required to be reduced, so that it takes more time for the head to accelerate to reach its nominal rotating speed and decelerates to stop.
In accordance with one aspect of the present invention, sheet counting apparatus comprises a set of rotatably mounted suction spindles mounted for movement past a stack of sheets to be counted, vacuum supply means connected to the spindles whereby as a suction spindle passes the stack, a vacuum is supplied to the spindle so that the topmost sheet is deflected from its initial position; and monitoring means for monitoring the number of deflected sheets, wherein the monitoring means monitors the degree of vacuum within the suction spindle passing the stack whereby the monitoring means increments a count on each occasion when the monitored vacuum exceeds a predetermined level threshold for a predetermined time.
In accordance with a second aspect of the present invention, a method of counting sheets using apparatus comprising a set of rotatably mounted suction spindles mounted for movement past a stack of sheets to be counted, whereby as a suction spindle passes the stack, a vacuum is supplied to the spindle so that the topmost sheet is deflected from its initial position comprises monitoring the degree of vacuum within a suction spindle passing the stack and incrementing a count on each occasion when the vacuum exceeds a predetermined level threshold for a predetermined time.
We have found that the problems described above can be overcome by considering the time during which the monitored vacuum exceeds the predetermined level threshold. Thus, it is not sufficient for the vacuum just to exceed the threshold in order to count a sheet but that condition must be maintained for the predetermined time.
The predetermined time may be set by the operator and typically this will be determined in accordance with the expected rotational rate of the spindles. In other examples, the predetermined time could be set, at least initially, automatically in accordance with the rotational speed of the spindles. Thus, the monitoring means may determine the rotation rate of the spindles and obtain the required predetermined time from a look-up table or the like.
In many cases, the spindle speed will vary during a count process and, of course, this will always happen at start-up when there is an acceleration in spindle speed and at the end of a count operation when there is a deceleration. These changes in speed will mean that the time during which the vacuum exceeds the predetermined level threshold will vary even for sheets of exactly the same type and condition. Consequently, in the preferred case, the predetermined time is varied. This may be achieved by monitoring the rotational speed of the spindles and/or by computing a rolling average of a predetermined number of previous times during which the monitored vacuum exceeded the predetermined level threshold.
A typical number of such predetermined times will be eight and, of course, in order to avoid erroneous values being utilized, the times could be compared with a minimum threshold with only those times which exceed this minimum threshold being used to compute the rolling average. This will then eliminate problems due to noise and the like.
In addition to varying the predetermined time, the predetermined level threshold could also be varied as described in more detail in EP-A-0616300. This then overcomes problems which may be encountered if the vacuum level changes, particularly reduces, for reasons such as porosity of the notes, and the reduction in force with which the stack of sheets is fed towards the suction spindles.